The invention concerns a vehicle driving arrangement with at least two axes, each having at least one driven wheel that can be acted upon with a torque of a motor, the motor/motors of each axis being arranged in a supply line, the supply lines being parallel to each other and connected with a pump device, and a valve arrangement being arranged in at least one supply line.
In certain operating situations, a problem occurs in connection with such hydraulic driving arrangements. It can, for example, appear during downhill driving, when the load acting on the front axis is larger than that acting on the rear axis. The driving motors then work as pumps, producing a pressure, which can be higher than the pressure of the pump device. Due to the different frictional contact of the wheels with the ground, caused by the different loads, it may now happen that the flow direction at the motors of a less loaded axis changes and the connected wheels start turning against the driving direction. To avoid this so-called backspin effect, some vehicle driving arrangements are provided with corresponding anti-backspin arrangements.
Such a vehicle driving arrangement is known from U.S. Pat. No. 5,199,525. Between the pump device and motor (rear wheel) that is critical with respect to back-spin effect, this driving arrangement has a pressure controlled valve arrangement. When a pressure difference of the supply line of the rear wheel reaches a preset value, which is set via a spring, the valve changes its position. In this position, the pressure admission of the rear wheel motor is interrupted by the pump device, and is now idling. This displacement of the valve arrangement occurs, for example, when the vehicle is driving forward downhill, and the pressure supplied by the motors of the front axis is higher than the pressure, with which the pump device acts upon the motors. In such a situation, the pressure controlled valve arrangement prevents the rear wheel from spinning against the driving direction, as soon as there is not sufficient frictional contact between wheel and ground (back-spin effect).
In such a driving arrangement the risk that a back-spin effect appears is effectively reduced, however, at the expense of the braking power of the rear wheel, which, in a corresponding situation merely runs along freely. After the position change of the valve arrangement, the reduction of the number of driven wheels on the motors of the remaining driven wheels may additionally cause a pressure increase. This causes an instantaneous increase in the speed.
The invention is based on the task of improving the operation of a hydraulically driven vehicle.
With a driving device as mentioned in the introduction, this task is solved in that the valve arrangement in one supply line has a pressure reducing valve that limits the pressure difference over the other supply line to a maximum value.
Firstly, this ensures an arrangement, which can prevent the spinning in the opposite direction of the driving direction of the wheels of an axis, which is liable to having a back-spin effect. In this connection, the arrangement works in a purely hydro mechanical mode, which reduces the costs, and thus also enables an economically sound use in simple vehicles. With a driving device according to the invention, a braking power can still be generated by the axis liable to having a back-spin effect, also when the anti-back-spin function is activated. In such a situation, this provides the vehicle with a higher braking power from the drive, which again improves the operating convenience and the safety of the vehicle. The pressure over the other axis remains substantially unchanged. Thus no instantaneous speed increase occurs.
It is advantageous that, in dependence of a predetermined frictional contact between the wheel driven by a motor, which is submitted to the limited pressure difference, and a ground, the maximum value is chosen so that during the anticipated operation a slip between the wheel and the ground will not occur. In this way, the braking power of the axis liable to back-spin is available for all imaginable operating conditions, also when the anti-back-spin function is activated. Thus, for the anticipated use of the vehicle, for example with regard to ground gradients and ground properties, a good operating convenience and a sufficient safety are ensured under all circumstances.
It is advantageous that the pressure difference of a supply line can only be limited, when in connection with at least one motor the pressure from the pump device, which acts upon this motor, is lower than the pressure, which is supplied by the motor. Thus, it is ensured that the pressure difference of a supply line is only limited, when there is a risk of a back-spin effect on the corresponding axis.
During the rest of the operation of the driving device, the full pressure difference is available, which ensures an improved utilisation of the pressure of the pump device, which again reduces the operating costs.
Further, it is advantageous that the valve arrangement limits the pressure in one direction over the pressure reducing valve to the maximum value, and in the other direction passes it on unchanged. In this way, it is ensured that the pressure reduction by the valve arrangement only occurs in one flow direction. In the opposite flow direction, for example when reversing, a motor of the own supply line is connected downstream of the valve arrangement, the motor thus being acted upon by the full pressure available. Thus, the highest possible pressure of the at least one motor keeps the pressure difference of this supply line, which can cause the back-spin effect, at the lowest possible level. Further, this causes that during normal operation the full pressure is available for driving the vehicle, which again reduces the operating costs.
Additionally, it is advantageous that the valve arrangement has a non-return valve arranged in parallel with the pressure reducing valve and opening in a flow direction, in which a pressure reduction shall not take place. In such an arrangement, all components of the complete valve arrangement have a simple design. This firstly reduces the costs of the driving arrangement, and secondly, the components are less susceptible to faults, which ensures a high operating reliability.
Preferably, in connection with forward driving, the pressure difference over the supply line of the rear axis in the driving direction (the rear axis of the vehicle) can be limited to the maximum value by the valve arrangement. As, in connection with forward driving downhill, the normal force acting upon the ground via the rear axis and its wheels is lower than the one acting via the front axis of the vehicle, the risk of a back-spin effect mainly exists for the rear axis. Thus, the limitation of the pressure difference over the supply line of the rear axis means a substantial reduction of the risk of back-spin.
In another preferred embodiment, the pressure difference over the supply line of the rear axis during reversing (the front axis of the vehicle) can be limited by a second valve arrangement. Thus, additionally, the risk of the occurrence of a back-spin effect also in connection with reversing is substantially avoided.
It is advantageous that the pressure reduction valve is adjustable. In this way, the driving device can be adjusted to the anticipated operating conditions of each individual vehicle. Thus, an optimised braking power of the drive can be set in dependence of the risk of the occurrence of a back-spin effect.
It is advantageous that the pressure reduction valve is adjustable through the pump device. Through this measure, a constant braking power of at least one of the wheels of the axis in question is adjustable when the anti-back-spin function is active, independently of the pressure supplied by the pump device at the given time. Thus, the anti-back-spin function of the driving arrangement can be adjusted even more precisely to the anticipated operating conditions.
Further, it is advantageous that the pump device has a variable pump. The use of the variable pump enables the pressure admission on the remaining driving device in two directions with a small room requirement of the pump device. Thus, both room and weight are saved.
Further, it is favourable that two motors are arranged on one axis, the motors being acted upon with pressure via the supply line of the axis. Thus, also two wheels are driven on one axis, which ensures a stable and uniform driving of the vehicle.
Preferably, both motors of an axis are arranged in parallel with each other in the supply line. The parallel connection of the motors causes that the fluid flow through the motor with the smaller resistance is larger than that through the motor with the larger resistance. In this way, for example in connection with curve driving, the fluid is distributed according to the need of both motors of an axis, which makes the driving via both wheels more uniform and thus ensures more stable driving qualities.
In another preferred embodiment, both motors of an axis are arranged in series in the supply line. The series connection of the two motors causes that the fluid flow through both motors is the same. Thus, the pressure is distributed on the resistances of the individual motors. If one of the driven wheels starts spinning, this wheel will only receive a relatively low pressure. The vehicle can thus be driven via the wheel of the other motor, at which a pressure corresponding to its resistance is still available.